Efficacy of tigecycline monotherapy versus combination therapy with other antimicrobials against carbapenem-resistant Acinetobacter baumannii sequence type 2 in Heilongjiang Province.

BACKGROUND Selecting alternative antibiotic combinations as treatment options may help successfully manage carbapenem-resistant Acinetobacter baumannii (CRAB). This study aimed to determine the synergistic effects of tigecycline (TIG) monotherapy versus combination therapy with other antimicrobials against CRAB. METHODS After performing biochemical identification assays, we detected oxacillin-hydrolyzing (OXA)-type carbapenemase genes in 35 CRAB isolates. The minimum inhibitory concentrations (MICs) and interactions of the test drugs were determined using the checkerboard assay with TIG, colistin (CST) and meropenem (MEM). Static time-kill assays were conducted to validate the synergistic effects of the most efficacious combination. RESULTS The chromosomal gene, blaOXA-51-like, was tested among all isolates, blaOXA-23-like and blaOXA-24-like were present in 91.4% and 25.7%, respectively. In the checkerboard assay, the combination of TIG and MEM displayed the highest rate of synergy (30.5%) against the 35 isolates. In contrast, the TIG-CST combination showed a higher indifference interaction rate (36.1%) than that of the TIG-MEM (16.7%) combination. Antagonism appeared in one isolate for the TIG-CST combinations. The static time-kill assays confirmed the superior synergistic effect of CST against the CRAB isolates. CONCLUSIONS TIG combined with CST exhibited early synergistic activity that was not sustained beyond 12 h. TIG combination therapy can only be recommended when other optimized therapeutics are unavailable.